Beurre maître d'hôtel is a fancy French way to say "butter with herbs". It's a special kind of butter that has herbs mixed in, often parsley and a little bit of lemon juice. The butter gets all mixed up with the herbs and the lemon juice so it tastes really special and delicious. You might have seen it served in a fancy restaurant or at a fancy event. It's usually used to put on top of meat or fish, kind of like a sauce. People like it because it makes their food taste really yummy and gives it a fancy touch.